Can't find drain under tile for a planned bathroom

I bought a new house after a family backed out of the deal with the contractor one week prior to its completion. The people that backed out decided to have one of the bathrooms converted to a dog room. The contractor told me that he couldn't give me the prints but everything was inplace to convert it back to the planned bathroom.

I've opened the wall and found the hot and cold water capped off. I've removed the tile from the floor, but can't find the drain. I've dug into the concrete about 1.5 inches where I believe the drain should be, but can't find it.

Since the contractor is on vacation and I want to start this renovation now, can anyone tell me how contractors usually leave a drain in this type situation. How deep? Where?

The bathroom was designed for a 60 inch tub, I want to install a shower vice a tub, but can't make any decisions until I find the drain.